154 



TESTING THE MICROSCOPE. 



formation of a sharply defined disc of light somewhat clearer at 

 the edge ; whilst increasing the distance of the eye-piece always 

 produces the opposite appearance. The same effect is also obtained 

 within the limits here considered, if, for the displacements of the 

 eye-piece, are substituted corresponding displacements of the 

 object, instead of moving the eye-piece nearer or further with 

 respect to the objective, the object is brought nearer or further, 

 or the body-tube is lowered or raised. Where, therefore, an ap- 

 proximation produces more fog, or increasing the distance produces 

 a well-defined circle of light, there is over-correction, and where 

 the converse phenomena appear, under-correction. 



With these effects of spherical aberration, which are clearly 

 apparent to the eye, are associated those of chromatic aberration, 

 which, as a rule, further increase the contrasts. In consequence 



of this, the fog appears bluish, and the disc of light has a greenish- 

 yellow border, red at the extreme edge, which appears, up to a 

 certain limit, the more decided the greater its diameter. The 

 distinction between under-corrected and over-corrected aberration 

 is thus still further facilitated. 



In certain micrographic works is added, as a further criterion 

 of the two opposite aberrations, the fact that the fine details of the 

 image (for instance, the window-frame, when mercury -globules are 

 used, or the margins of small apertures in an otherwise dark field 

 of view) disappear at once on the approximation of the object, but 

 only slowly on its withdrawal, if the objective is under-corrected ; 

 and conversely in an over-corrected instrument. Thus generally 

 stated, this assertion is inaccurate ; we have submitted the matter 

 to the test of experiment. In one special case, as shown in Fig. 

 90, precisely the converse took place. For it is clear that a dis- 



